Prompt for filename when saving a message

Replace the immediate save-to-~/ID.eml with a filename prompt (default
ID.eml) consistent with the part-save UX. Enter confirms, Esc cancels.
Removes the now-unused saveMessage/saveCmd helpers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

msgs.go

@@ -54,9 +54,9 @@ func loadMessageCmd(id string) tea.Cmd {
 	}
 }
 
-func saveCmd(id, content string) tea.Cmd {
+func saveMessageAsCmd(content, name string) tea.Cmd {
 	return func() tea.Msg {
-		path, err := saveMessage(id, content)
+		path, err := savePart([]byte(content), name)
 		if err != nil {
 			return saveErrMsg{err}
 		}